Translocation of components within the cell is critical for maintaining cell structure and function. Cellular components such as proteins and membrane-bound organelles are transported along well-defined routes to specific subcellular compartments. Intracellular transport mechanisms utilize microtubules which are filamentous polymers that serve as tracks for directing the movement of molecules. Molecular transport is driven by the microtubule-based motor proteins, kinesin and dynein. These proteins use the energy derived from ATP hydrolysis to power their movement unidirectionally along microtubules and to transport molecular cargo to specific destinations.
Kinesin defines a ubiquitous, conserved family of over 50 proteins that can be classified into at least 8 subfamilies based on primary amino acid sequence, domain structure, velocity of movement, and cellular function. (Reviewed in Moore, J. D. and Endow, S. A. (1996) Bioessays 18:207-219; and Hoyt, A. M. (1994) Curr. Opin. Cell Biol. 6:63-68.) The prototypical kinesin molecule is involved in the transport of membrane-bound vesicles and organelles. This function is particularly important for axonal transport in neurons. Protein-containing vesicles are constantly transported from the neuronal cell body along microtubules that span the length of the axon leading to the synaptic terminal. Failure to supply the synaptic terminal with these vesicles blocks the transmission of neural signals. In the fruit fly Drosophila melanogaster, for example, mutations in kinesin cause severe disruption of axonal transport in larval nerves which leads to progressive paralysis (Hurd, D. D. and Saxton, W. M. (1996) Genetics 144:1075-1085). This phenotype mimics the pathology of some vertebrate motor neuron diseases, such as amyotrophic lateral sclerosis (ALS). In addition to axonal transport, kinesin is also important in all cell types for the transport of vesicles from the Golgi complex to the endoplasmic reticulum. This role is critical for maintaining the identity and functionality of these secretory organelles.
Members of the more divergent subfamilies of kinesin are called kinesin-related proteins (KRPs), many of which function during mitosis in eukaryotes as divergent as yeast and human (Hoyt, supra). Some KRPs are required for assembly of the mitotic spindle. In vivo and in vitro analyses suggest that these KRPs exert force on microtubules that comprise the mitotic spindle, resulting in the separation of spindle poles. Phosphorylation of KRP is required for this activity. Failure to assemble the mitotic spindle results in abortive mitosis and chromosomal aneuploidy, the latter condition being characteristic of cancer cells. In addition, a unique KRP, centromere protein E, localizes to the kinetochore of human mitotic chromosomes and may play a role in their segregation to opposite spindle poles.
The prototypical kinesin molecule is a heterotetramer comprised of two heavy polypeptide chains (KHCs) and two light polypeptide chains (KLCs). The KHC subunits are typically referred to as "kinesin." KHC is about 1000 amino acids in length, and KLC is about 550 amino acids in length. Two KHCs dimerize to form a rod-shaped molecule with three distinct regions of secondary structure. At one end of the molecule is a globular motor domain that functions in ATP hydrolysis and microtubule binding. Kinesin motor domains are highly conserved and share over 70% identity. Beyond the motor domain is an .alpha.-helical coiled-coil region which mediates dimerization. At the other end of the molecule is a fan-shaped tail that associates with molecular cargo. The tail is formed by the interaction of the KHC C-termini with the two KLCs.
The nematode Unc-104 kinesin-like protein defines a distinct kinesin subfamily whose members may function monomerically (Moore and Endow, supra). Members of this subfamily are important for synaptic transport and mitochondrial translocation and are characterized by movement at relatively high velocities of about 40 to 90 microns/minute. Nematodes with mutations in the Unc-104 gene exhibit defects in locomotion and feeding behaviors, and at the molecular level, in synaptic vesicle transport.
The discovery of a new kinesin-like motor protein and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of cancer, neurological disorders, and disorders of vesicular transport.